Pro Sites and enabling specific plug-ins

I’m new to Pro Sites and slowly getting my head around everything. But there’s one thing I want to do and cannot quite see how to do it.

I want to offer my users several paid levels, but then have the option to add extra bolt-ons for an additional fee.

For example, using the levels from the documentation, say I have a user that’s at the top level ‘Elite’ – I want to allow them to add say, E-commerce or maybe a forum.

I could just create a level that’s ‘Elite plus E-commerce’ and ‘Elite plus forum’, but I’d also need ‘Elite plus E-commerce, plus forum’. Obviously this gets more and more complex as I add features!

Is there a way to add (by that I mean, allow my users to purchase) extras? If so, can I do this with Pro Sites or do I need something else? I was looking at the ‘Membership’ plugin, but I don’t think that does what I want.